Signals between electronic components in computers and other appliances have traditionally been carried by copper wire or other metal interconnects. Increasing speeds of digital electronics have pushed this type of signal carriage to its limits, even between components on a single integrated circuit chip. Optical signals can carry vastly more data than traditional electrical signals in metallic conductors, and this has led to a need for optical communication devices on the scale of integrated circuit chips. Silicon photonic devices have attracted much attention for both on-chip and inter-chip optical signals because silicon is inexpensive and compatible with silicon CMOS component fabrication. High-efficiency silicon optical modulators, filters and waveguides have been built directly on silicon substrates. With multiplication gain, avalanche photodiodes (APDs) are ten times or more as sensitive as PIN photodiodes and are therefore attractive for low-power, high-sensitivity data links between silicon chips.